470 



NATURE 



\_Sept. 12, 18S9 



should be touched upon by the President, and I shall, as far as 

 in me lies, follow this procedure. 



This year is the jubilee of the practical introduction of photo- 

 graphy by Daguerre and Fox Talbot, and I have thought I 

 might venture to take up your time with a few remarks on the 

 effect of light on matter. I am not going into the history of 

 photography, nor to record the rivalries that have existed in 

 regard to the various discoveries that have been made in it. A 

 brand-new history of photography, I dare say, would be inter- 

 esting, but I am not the person to write one ; and I would 

 refer those who desire information as to facts and dates to 

 histories which already exist. In foreign histories perhaps we 

 English suffer from speaking and writing in a language which 

 is not understanded of the foreign people ; and the credit of 

 several discoveries is sometimes allotted to nationalities who 

 have no claim to them. Be that as it may, I do not propose 

 to correct these errors or to make any reclamations. I leave 

 that to those whose leisure is greater than mine. 



I have often asserted, and I again assert, that there should 

 be no stimulus for the study of science to be compared to photo- 

 graphy. Step by step, as it is pursued, there will be formed 

 a desire for a knowledge of all physical science. Physics, 

 chemistry, optics, and mathematics are all required to enable 

 it to be studied as it should be studied ; and it has the great 

 advantage that experimental work is the very foundation of it, 

 and results of some kind are always visible. I perhaps am 

 taking an optimist view of the matter, seeing there are at least 

 25,000 living facts against my theory, and perhaps not i per 

 cent, of them in its favour. I mean that there are at least 

 25,000 persons who take photographs, and scarcely i per cent, 

 who know or care anything of the "why or wherefoie" of the 

 processes, so far as theory is concerned. If we call photo- 

 graphy an applied science, it certainly has a larger number 

 who practise it, and probably fewer theorists, than any other. 



He would be a very hardy man who would claim for Niepce, 

 Daguerre, or Fox Talbot the discovery of photographic action on 

 matter. The knowledge that such an action existed is probably 

 as old as the fair-skinned races of mankind, who must have 

 recognized the fact that light, and particularly sunlight, had a 

 tanning action on the epidermis, and the women then, as now, 

 no doubt took their precautions against it. As to what change 

 the body acted upon by light underwent it need scarcely be 

 said that nothing was known, and perhaps the first scientific 

 experiment in this direction was made rather more than a 

 hundred years ago by Scheele, the Swedish chemist, who found 

 that when chloride of silver was exposed to light chlorine was 

 given off. It was not till well in the forties that any special 

 attention was given to the action that light had on a variety of 

 different bodies ; and then Sir John Herschel, Robert Hunt, 

 Becquerel, Draper, and some few others carried out experiments 

 which may be termed classical. Looking at the papers which 

 Herschel published in the Philosophical Transactions and else- 

 where, it is not too much to say that they teem with facts which 

 support the grand principle that without the absorption of 

 radiation no chemical action can take place on a body ; in other 

 words, we have in them experimental proofs of the law of the 

 conservation of energy. Hunt's work, " Researches on Light," 

 is still a text-book to which scientific photographers refer, and 

 one is sometimes amazed at the amount of experimental data 

 which is placed at our disposal. The conclusions that Hunt 

 drew from his experiments, however, must be taken with caution 

 in the light of our present knowledge, for they are often vitiated 

 by tbe idea which he firmly held, that radiant heat, light, and 

 chemical action, or actinism, were each of them properties, 

 instead of the effects, of radiation. Again, we have to be careful 

 in taking seriously the experiments carried out with light of 

 various colours when such colours were produced by absorbing 

 media. It must be remembered that an appeal to a moderately 

 pure spectrum is the only appeal which can be legitimately made 

 as to the action of the various components of radiation, and even 

 then the results must be carefully weighed before any definite 

 conclusion can be drawn. No photographic result can be con- 

 sidered as final unless the experiments be varied under all the 

 conditions which may possibly arise. Coloured media are 

 dangerous as enabling trustworthy conclusions to be drawn, 

 unless the characters of such media have been thoroughly 

 well tested and the light they transmit has been measured. An 

 impure spectrum is even more dangerous to rely upon, since the 

 access of white light would be sure to vitiate the results. 



Perhaps one of the most puzzling phenomena to be met with 



in photography is the fact that the range of photographic action 

 is spread over so large a portion of the spectrum. The same 

 difficulty of course is felt in the matter of absorption, since the one 

 is dependent on the other. Absorption by a body we are ac- 

 customed, and indeed obliged by the law of the conservation of 

 energy, to consider as due to the transference of the energy of 

 the ether wave-motion to the molecules and atoms comprising 

 the body by increasing the vibrations of one or both. 



In the case where chemical action takes place we can scarcely 

 doubt that it is the atoms which in a great measure take up the 

 energy of the radiation falling on them, as chemical action is 

 dependent on the liberation of one or more atoms from the 

 molecule, whilst, when the swings of the molecules are increased 

 in amplitude, we have a rise in temperature of the body. I shall 

 confine the few remarks I shall make on this subject to the case 

 of chemical action. The molecule of a silver salt, such as bromide 

 of silver, chemists are wont to look upon as composed of a 

 limited and equal number of atoms to form the molecule. When 

 we place a thin slab of this material before the slit of the 

 spectroscope we find a total absorption in the violet and ultra- 

 violet of the spectrum, and a partial absorption in the blue and 

 green, and a diminishing absorption in the yellow and red. A 

 photographic plate containing this same salt is acted upon in 

 exactly the same localities and in the same relative degree as 

 where the absorption takes place. Here, then, we have an 

 example of, it may be, the vibrations of four atoms, one 

 of which at least is isochronous, or partially so, with the 

 waves composing a large part of the visible spectrum. 

 The explanation of this is somewhat obscure. A mental 

 picture, however, may help us. If we consider that, owing 

 to the body acted upon being a solid, the oscillations of 

 the molecules and atoms are confined to a limited space, it 

 probably happens that between the times in which the atoms 

 occupy, in regard to one another, the same relative positions, 

 the component vibrations of, say, two of the atoms vary con- 

 siderably in period. An example of what I mean is found in a 

 pendulum formed of a bob and an elastic rod. If the bob be 

 made to vibrate in the usual manner, and at the same time the 

 elastic rod be elongated, it is manifest that we have a pendulum 

 of ever-varying length. At each instant of time the period of 

 vibration would differ from that at the next instant, if the 

 oscillations were completed. It is manifest that increased 

 amplitude would be given to the pendulum swings by a series 

 of well-timed blows differing very largely in period ; at the same 

 time there would be positions of the pendulum in which some 

 one series of well-timed blows would produce the greatest effect. 

 In a somewhat similar manner we should imagine that the 

 ethereal waves should produce increased amplitude in the swing 

 of the atoms between very wide limits of period, and, further, 

 that there should be one or more positions in the spectrum when 

 a maximum effect is produced.* I would here remark that the 

 shape of the curves of sensitiveness, when plotted graphically, 

 of the different salts of silver to the spectrum have a marked 

 resemblance to the graphically drawn curves of the three colour- 

 sensations of the normal eye, as determined by Clerk Maxwell. 

 May not the reason for the form of the one be equally applicable 

 for the other ? I only throw this out as evidence, not conclusive 

 indeed, that the colour-sensitiveness of the eye is more probably 

 due to a photographic action on the sensitive retina than to a 

 merely mechanical action. That this is the case 1 need scarcely 

 say has several times been propounded before. 



The ease with which a silver salt is decomposed is largely, if 

 not quite, dependent on the presence of some body which will 

 take up some of the atoms which are thrown off from it. For 

 instance, in chloride of silver we have a beautiful example of 

 the necessity of such a body. In the ordinary atmosphere the 

 chloride is, of course, coloured by the action of light ; but if it 

 be carefully dried and purified, and placed in a good vacuum, it 

 will remain uncoloured for years in the strongest sunlight. 

 In this case the absence of air and moisture is sufficient to 

 prevent it discolouring. 



If in the vacuum, however, a drop of mercury be introduced, 

 the coloration by light is set up. We have the chlorine liberated 

 from the silver and combining with the mercury vapour, and a 

 minute film of calomel formed on the sides of the vessel. 



Delicate experiments show that not only is this absorbent 

 almost necessary when the action of light is so strong or 



' The effect of perfect and nearly perfect synchronism of one oscillation 

 upon another is also to be found exemplified in my "Treatise on Photo- 

 graphy" (" Text-book of Science Series "). 



